At 100°C, water can exist in both liquid and gaseous form. This is because 100°C is the boiling point of water, which means that when water is heated to this temperature, it starts changing from liquid state to gaseous state. The heat required to change water from liquid to gas is called the latent heat of vaporization. Therefore, at 100°C, water can exist as both water vapor (gaseous state) and liquid water.